The CO2 Record in Plant Fossils

Plant fossils obtained from sedimentary rocks and peat deposits are a relatively new tool being used to unravel Earth’s carbon dioxide (CO2) history. Tiny pores on plant leaves and needles called stomata regulate carbon dioxide absorption and water vapor release. Stomata numbers decrease during times of high atmospheric CO2, and increase when atmospheric CO2 is low.

Nature’s CO2 meter:

A standardized way of counting stomata– called the stomatal index ( SI [%] )– has been found to be a good way to estimate the CO2 content of the atmosphere when the plant was alive. The SI-CO2 relationship varies according to plant species, habitat altitude, and other factors.

Correlation charts are constructed using modern plant specimens by determining their SI numbers and corresponding CO2 concentrations. When SI and CO2 ranges are fully characterized for a plant species, the charts are used as to estimate CO2 levels for related species in the geologic past.

To determine plant ageCarbon14 methods are usually used to about 40,000 years ago. For older material, other dating methods are used.

Because plant stomata numbers do not change after the leaves or needles fall from the parent plant, they make a good indicator or proxy of atmospheric CO2 in Earth’s past. What they show is that the popular belief that CO2 levels prior to the Industrial Revolution were a steady 280 ppm (parts per million) may be incorrect.

As illustrated below, studies of stomata for recent and fossilized plants show that atmospheric CO2 levels over the last 15,000 years have been higher and much more variable than previously supposed. Much of what we think we know about CO2 levels of the past 800,000 years is based on the ice core record.

Recent stomata studies show that CO2 was more variable and the average CO2 concentrations have been significantly higher during our Holocene interglacial period (last 11,000 years) than are indicated by the ice core record.

Recent stomata studies show that CO2 was more variable and the average CO2 concentrations have been significantly higher during our Holocene interglacial period (last 11,000 years) than are indicated by the ice core record.

Read below for more details.

The Ice Core Record

Ice cores obtained by drilling into permantent ice caps in Antarctica and Greenland have been the most important way to determine past levels of carbon dioxide— however, recent stomata studies show that the ice core record may be misleading in several important respects.

For example, when ice cores are crushed to extract the gases from trapped air bubbles to determine CO2 content, there is an assumption made that ice bubbles preserve an accurate record of the Earths CO2 history. However, the chemical composition of ice bubbles undergo changes that may distort this record.

Accumulating ice layers can take a century or more to become buried deep enough to be isolated from the atmosphere, which at the South Pole occurs at a depth of approximately 120 m. The resulting heat and pressure causes gas exchange between ice layers, which modifies the chemistry of ice air bubbles. At burial depths of between 900 and 1200 meters the pressure is so great that air bubbles in ice disappear and the gases recombine with liquids and ice crystals. Such processes tend to smooth away variability in the ice record and may also make CO2 levels appear lower than they really were, obscuring much of the resolution pertaining to CO2 variability (1-4).

“Liquid water is common in polar snow and ice, even at temperatures as low as -72C, (and) in cold water, CO2 is 70 times more soluble than nitrogen and 30 times more soluble than oxygen– guaranteeing that the proportions of the various gases that remain in the trapped, ancient air will change. Moreover, under the extreme pressure that deep ice is subjected to — 320 bars, or more than 300 times normal atmospheric pressure — high levels of CO2 get squeezed out of ancient air.”

Although the ice core record represents a very nice overall view of temperature and CO2 trends over many thousands of years, their reliability for resolving details over timescales of decades– or in some cases several centuries– is limited. Nonethess, these data are used as the principle evidence to show that CO2 levels in excess of 300 parts per million are unprecedented in all of human history and a cause for concern.

The Presumption of CO2 Stability

The records of CO2 and Temperature over the last 15,000 years (but without the stomata CO2 record) appear in Figure 3. Except for the South Pole Air Flask CO2 measurements, all other data shown (including temperature) are from ice cores.

Figure 3. The CO2 record for the past 15,000 is comprised of ice cores, mostly. These came from Law Dome and Dome C in Antarctica. Since 1957 the CO2 record at the South Pole has been by analyzing Air Flask samples. (see larger image). By convention, “Years B.P.” (Before Present) begins 1950 AD, which is why later years are “negative.”

According to the Dome C and Law Dome ice cores, for nearly 15,000 years prior to the Industrial Revolution CO2 has remained below 280 ppm(parts per million), while only the youngest part of the Law Dome cores (after 1900 AD) show CO2 concentrations higher than 300 ppm.

The youngest CO2 data, is not based on ice cores but on South Pole Air Flask samples– which consistently show CO2 higher than 300 ppm. The point in time useful for considering what CO2 concetrations really were before humans started to burn fossil fuels is at the start of the Industrial Revolution– about 1750 AD. A key assumption is that pre-Industrial CO2 concentrations were less than 280 ppm and that everything above that is caused by humans. This assumption, however, is not without problems, although seldom discussed.

Basis for the Estimate of Pre-Industrial CO2

Figure 4.

The Industrial Revolution started in Europe in the mid 1700’s. The time before is referred to as “Pre-Industrial” time.

Because reliable CO2 air tests were not being performed until the 1800’s, the presumed CO2 concentration in 1750 is 280 ppm, based largely on ice core data and early work by G.S. Callendar.

In the 1800’s direct air CO2 measurements were performed by various researchers. Interestingly, the CO2 levels reported by them were mostly in excess of 300 ppm. For reasons that are unclear, only a few of these tests were considered valid by G.S. Calendar (1898-1964)– the grandfather of the theory of man-made global warming. Today, the remaining data are largely ignored, although a few commentators like E. Beck and Z. Jaworoski suggest the data–some compiled by Nobel Prize laureates– are generally valid and were inappropriately dismissed (4, 21) .

Callendar claimed humans had increased CO2 concentrations in the atmosphere by burning fossil fuels, and had thereby changed the atmosphere from 274 ppmv to 325 ppmv by 1935– resulting in a 18.3 percent increase which had caused the global surface temperature to rise 0.33 deg. C (5). However, CO2 data available at the time showed concentrations ranged between 250 ppm and 550 ppm (Figure 4). Callendar has been accused of cherry-picking data from a sampling of 19th century averages, using 26 that supported his ideas, but rejecting 16 that were higher than his assumed low global average, and 2 that were lower (6).

Despite numerous 19th century air measurements showing +300 ppm CO2 levels, and despite the fact that many of the youngest ice cores showed higher than expected CO2 values and so were shifted forward 90-100 years from previously-established dates so that they would match the more elevated CO2 levels of 20th century air samples, the ice core record is today generally used to represent pre-1957 CO2 concentrations. The Intergovernmental Panel on Climate Change (IPCC) places the pre-industrial concentration of CO2 in the atmosphere at 280 ppm, based largely on the ice core record, although this has never been otherwise substantiated (7).

When systematic air readings began in 1957 AD, CO2 air values were about 315 ppm. Today, CO2 concentrations are about 384 ppm. Current estimates of the anthropogenic (man-made) component of atmospheric CO2 range between 4% (9) and 25% (the latter assumes Pre-Industrial levels were 280 ppm, and assumes everything over that today is man-made). The problem with the 280 ppm baseline figure is that increasing evidence suggests this figure may be too low.

CO2 levels exceeding 300 ppm, we are told, are unnatural and unprecedented, but available 19th century CO2 air data and studies of plant stomata suggest another side to the story.

Fiction:

“The recent rate of change is dramatic and unprecedented; increases in CO2 never exceeded 30 ppm in 1 kyr – yet now CO2 has risen by 30 ppm in just the last 17 years.”

The Last 15,000 Years– Reconsidered

Studies of plant stomata show that the currently-held view of predominantly stable CO2 levels (260-280 ppm) before the Industrial Revolution (1750 AD, i.e. 200 years B.P.) may be an inaccurate view. CO2 levels appear to have regularly exceeded 280 ppm– the averageof CO2 concentrations across the Holocene interglacial period (last 11,000 years) appears to have been approximately 305 ppm (see ref. 10-20).

Contrary to the prevailing notion of CO2stability, CO2 swings of 20-50 ppm or more over timespans of 500-1000 years appear to be the norm– not the exception.

Figure 5. Illustrated here are results from recent stomata studies which show that CO2 was more variable and the average CO2 concentrations have been significantly higher during our Holocene interglacial period (last 11,000 years) than are indicated by the ice core record. A precipitous drop in CO2 during the “Younger Dryas” was captured nicely by the stomata record, but missed by the CO2 record in ice cores. (larger image).

Stomata researchers regard the plant stomata proxy as a reliable means to measure CO2 levels in the geologic past, including the Holocene interglacial period, which spans the period from about 12,000 years ago and continues to the present.

Data from various stomata studies (ref. 10-20) show CO2 concentrations over the last 11,000 years varied between 260 and 340 ppm (average: 305 ppm). In contrast, the Dome C ice core record shows no significant variability and considerably lower overall CO2 levels (average: 270 ppm).

A sharp CO2 decline is indicated between 11,500 to 12,800 B.P., coinciding with an abrupt cooling phase, known as the “Younger Dryas” (Figure 5 ). While this event is obscured in the Antarctic Dome C ice core CO2 record, it shows up clearly in the stomata CO2 record.

Based on these stomata data, the conventional Pre-Industrial baseline of 280 ppm may be understated by about 25 ppm. In other words, 24% of the presumed 105 ppm Industrial Era CO2 increase may in fact be a result of bias and poor resolution of CO2 variability in the ice cores.

While the stomata data show higher values of CO2 than do pre-1900 ice data, they generally agree with the very youngest part of the Law Dome ice data (1900-1957 AD) and also with the contemporary S. Pole Air Flask CO2 record (actual air samples) begun in 1957 and continuing today. In other words, stomata results agree with the data that are least susceptible to distortion and diffusion errors.

The stomata record offers important evidence to challenge the notion that variations in CO2 levels of 20-50 ppm over timespans of less than 1000-years are “unprecedented” or that Pre-Industrial CO2 concentrations never went above 300 ppm– both may, in fact, have been normal.

Putting Things in Perspective

New studies of plant stomata add important information about natural CO2 variations in Earth’s atmosphere. Such studies show that natural variations in CO2 are more dramatic than we have been led to believe, and that CO2 levels which regularly rise past 300 ppm may be the norm– not the exception– during the last 11,000 years. Natural CO2 levels up to 340 ppm are suggested during this time, challenging claims that 300 ppm represents a CO2 threshold which is both “unprecedented” and un-natural in our recent climate history.

In reality, the actual amount of human additions to CO2 over the past 250 years is more of an academic issue than a practical one, as the theory that human additions to atmospheric CO2 are the principle driver of Earth’s temperature changes, has not been proven. For example:

Temperatures in the mid troposphere (5 km up), where signals of greenhouse warming should be strongest, have actually declined since 2000. According to greenhouse theory, this should not be happening if CO2 increases are the primary cause of global warming.

As the case for a CO2 problem looks increasingly uncertain it is appropriate to question climate projections and computer models on global warming to ensure that we are not basing important and expensive decisions on information that currently may be no more meaningful than answers given by a magic 8-ball.

Given the many complexities of clouds, ocean sinks, cosmic influences, and historical uncertainties, it is clear that our understanding of CO2 levels and climate cycles is incomplete. A new piece to this puzzle comes from simple plant fossils, which hold important clues about Earth’s dynamic climate past– and future.

8) The ice-core man “Once upon a time, and for millennia before then, carbon dioxide levels in the atmosphere were low and stable…”; by Lawrence Solomon for National Post, May 23, 2007; re-printed by canada.com.